Photoelectrophoretic imaging method employing a halogen containing suspension

ABSTRACT

THIS INVENTION RELATES TO A PHOTOELECTROPHORETIC IMAGING SYSTEM WHEREBY UPON THE ADDITION OF A HALOGEN TO THE IMAGING SUSPENSION IT IS POSSIBLE BY REGULATING THE POLARITIES OF THE ELECTRODES IN THE SYSTEM TO OBTAIN BOTH NEGATIVE AND POSITIVE IMAGES FROM A SINGLE SENSE INPUT.

March 7, 1972 J. a. WELLS 3,647,660

PHOTOELECTROPHORETIC IMAGING METHOD EMPLOYING A HALOGEN CONTAINING SUSPENSION Filed Dec. 19, 1969 INVENTOR;

BY aw ATTORNEY United States Patent PHOTOELECTROPHORETIC IMAGING METHOD EMPLOYING A HALOGEN CONTAINING SUSPENSION John B. Wells, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y. Filed Dec. 19, 1969, Ser. No. 886,534 Int. Cl. Blllk 5/00; G03g 13/00 US. Cl. 204-181 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a photoelectrophoretic imaging system whereby upon the addition of a halogen to the imaging suspension it is possible by regulating the polarities of the electrodes in the system to obtain both negative and positive images from a single sense input.

BACKGROUND OF THE INVENTION This invention relates to an imaging system and more specifically to a photoelectrophoretic imaging system.

In photoelectrophoretic imaging colored photosensitive particles are suspended in an insulating carrier liquid. This suspension is placed between at least two electrodes, subjected to a potential difference and exposed to a light image. Ordinarily, in carrying out the process, the suspension is placed on a transparent electrically conductive member in the form of a thin film and exposure is made through this member while a second generally cylindrically shaped biased electrode is rolled across the suspension. The particles are believed to bear an initial charge when suspended in the liquid carrier which causes them to be attracted to the transparent base electrode and, upon exposure, to change polarity so that the exposed particles migrate to the second or roller electrode thereby forming complemenetary images on each of the electrodes by particle subtraction. The process may be used to produce both polychromatic and monochromatic images. -In the latter instance a single color photoresponsive particle may be used in the suspension or a number of differently colored particles may be used all of which respond to the exposure radiation. Anextensive and detailed description of such a process may be found in U.S. Pat. Nos. 3,383,993; 3,384,488; 3,384,565; and 3,384,566, and are hereby incorporated by reference.

Although good quality images can be produced utilizing the above principles due to the nature of the process, the imaging particles tend to respond in one particular imaging mode. Thus, in a monochromatic system, acceptable imaging has been found to be generally restricted to a single sense process so that with positive input a negative image may be reproduced and vice versa.

It is, therefore, an object of this invention to provide an imaging system which will overcome the above-noted disadvantages.

It is a further object of this invention to provide a novel photoelectrophoretic imaging system capable of a high degree of flexibility.

Another object of this invention is to provide a novel photoelectrophoretic monochromatic imaging system.

It is still a further object of this invention to provide a novel photoelectrophoretic imaging composition.

Still a further object of this invention is to provide a one step imaging process capable of producing images of either optical sense regardless of the sense of the informational input.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with the present invention generally speaking by providing an imaging suspension of colored photoelectrophoretic imaging particles in an insulating carrier liquid. The imaging suspension further contains a halogen additive which together with the proper orientation of the electrode polarities makes it possible to produce high contrast images in both a positive and negative imaging sense. The suspension is interpositioned between at least two electrodes and subjected to an electrical field. The suspension is then selectively exposed to a reproducible image by a source of activating electromagnetic radiation. Generally speaking, the imaging-suspension is placed on a transparent, electrically conductive member or first electrode in the form of a thin film and exposure is made through this transparent member while contact is made with a second, imaging electrode. The photomigratory particles present in this suspension respond to the radiation to form a visible image pattern at one or both of the electrodes the images being complementary in nature. The imaging suspension employs intensely colored pigment particles the photosensitive pigment portion of which serves both as the colorant and as the photosensitive material. Additional photosensitive materials are not required thus providing a very expedient imaging process.

It has been determined that upon introducing a halogen into the photomigratory imaging suspension of the present invention that it is now possible upon proper orientation of the electrode polarities to produce high contrast, background free images in either a positive to negative or a positive to positive imaging mode. Heretonow, the materials used have substantially restricted the imaging capabilities of the photoelectrophoretic monochromatic imaging system. It has now been demonstrated that upon introducing into the imaging suspension a halogen additive, for example, iodine, that it is now possible to produce images of either sense in a one step process by simply changing the polarity of the bias on the electrodes in the system.

DESCRIPTION OF THE DRAWINGS The invention is more specifically defined in the accompanying drawing in which there is seen a transparent injecting electrode generally designated 1 which, in this instance, is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide. Tin oxide coated glass of this nature is commercially available under the trade name NESA glass. This electrode shall hereafter be referred to as the injecting or transparent electrode. Coated on the surface of the injecting electrode 1 is a thin layer 4 of the imaging suspension of the present invention comprising finely divided photosensitive pigment particles dispersed in an insulating carrier liquid. The term photosensitive, for the purposes of this application, refers to the properties of a particle which will migrate under the influence of an applied electric field when it is exposed to actinic radiation. A further detailed explanation of the apparent mechanism of the operation is disclosed in the above US. Pat. Nos. 3,384,565 and 3,384,566.

The imaging suspension will consist of specifically colored, finely divided photosensitive particles dispersed in an insulating carrier liquid or vehicle. Any suitable photosensitive pigment particle may be used such as disclosed in US. Pat. Nos. 3,384,565 and 3,384,566. As above stated, the pigments portion of the photomigratory particle provides both the photosensitivity and coloration for the respective particle. Any suitable insulating carrier liquid may be used in the course of the present in vention. Typical insulating carrier liquids include long chain saturated aliphatic hydrocarbons such as decane, dodecane, and tetradecane, kerosene fractions such as Sohio Odorless Solvents available from the Standard Oil Company of Ohio, Isopar G commercially available from the Humble Oil Co. of New Jersey and paraffin wax, molten beeswax and other molten thermoplastic materials, mineral oil, linseed oil, olive oil, marine oils such as sperm oil and cod liver oil, silicone oil such as dimethyl polysiloxane (Dow Corning Co.) fluorinated hydrocarbons such as Freon and mixtures thereof. The imagining suspension may also contain a sensitizer and/or binder for the pigment particles. To the imaging suspension of the present invention is added a halogen such as iodine, or bromine in amounts ranging from about 2 to about 200 ppm. Preferred concentrations range from about 6 to about 10 ppm.

Above the liquid suspension is passed a second or imaging electrode which, in this illustration, is represented as a roller having a conductive central core 11 connected to a power source 6. The core in this instance is covered with a layer 12 of material capable of blocking DC. current, such as paper, or polyurethane and will be referred to as a blocking layer. As indicated, the imaging or blocking electrode is connected to one side of potential source 6, while the opposite side is connected to the injecting electrode 1 through switch 7 so that when the latter is closed an electric field is applied across the suspension 4 between electrodes 1 and 5. The pigment suspension is exposed by way of the projection mechanism made up of light source 8, transparency 9 and a lens system 10. For purposes of this illustration a microfilm positive is used during the process. Although the blocking layer need not be used in the system, the use of such a layer is preferred due to the markedly improved results which it is capable of producing. A detailed description of the improved results and the types of materials which may be employed as the blocking layer may be found in US. Pat. No. 3,383,993. The blocking electrode 5 having a cylindrical configuration in the present illustration is rolled across the top surface of the injecting electrode 1 supporting the suspension 4 containing the photomigratory particles. Switch 7 is closed during the period of image exposure. Upon proper orientation of the electrode polarities the light exposure causes the exposed pigment particles suspended in the carrier liquidto migrate to the surface of electrode 5 while leaving behind a complementary image on the surface of the injecting electrode 1 of the particles not exposed. Upon reversing the polarities at which the field is applied the image sense is reversed while maintaining the same input information. It should be noted at this point of the discussion that although the blocking electrode in the present illustration is represented as a cylinder it may also take the form of a fiat plate electrode, as in the case of the illustrated injecting electrode, and the blocking electrode could be the optically transparent electrode and exposure made through it. Thus, it is to be understood that it is not intended that the structural arrangement of the apparatus represented by the illustration be restricted to the design as set out herein and all similar configurations which will satisfy the requirements of the present invention are contemplated. For example, the electrodes utilized may be cylindrically shaped thus providing for an expedient continuous process.

The pigment image produced need not necessarily be.

formed on the surface of an electrode but may in fact be formed on a removable paper substrate or sleeve superimposed on or wrapped around the blocking electrode or otherwise interpositioned between the electrodes at the site of imaging. The pigment image may then be fixed in place as for example by placing a lamination over its top surface such as by spraying with a thermoplastic composition, or by solvent evaporation. The image may also be transferred to the surface of a receiver substrate to which it may in turn be fixed. This would especially be desirable in the case Where the image is formed directly on the electrode surface. Such a transfer step may be carried out by adhesive pickoff techniques or preferably by electrostatic field transfer while the image is still wet. The blocking layer itself may be in the form of a removable sleeve in which instance it is simply replaced following imaging with a. similar material. When the image is formed on a substrate wrapped about or superimposed on the electrode itself it is only necessary to disengage the substrate from the electrode surface. The system herein described produces a high contrast monochromatic image with little or no background degradation in either a positive to negative or positive to positive imaging mode by a simple reversing of the polarities within the system.

Any suitable material may be used as the receiving substrate for the image produced such as paper or various transparent plastics such as Mylar (polyethylene terephthalate), Tedlar (polyvinylfiuoride) or cellulose acetate sheets, the latter particularly if it is desirable to produce a transparency suitable for image projection.

When used in the course of the present invention, the term injecting electrode should be understood to mean that it is an electrode which will preferably be capable of exchanging charge with the photosensitive particles of the imaging suspension when the suspension is exposed to light so as to allow for a net change in the charge polarity on the particle. By the term blocking electrode is meant one which is substantially incapable of injecting charge carriers into the above mentioned photosensitive particles thus substantially blocking D.C. current. The use of the blocking electrode serves to minimize particle oscillation in the system.

It is preferred that the injecting electrode be composed of an optically transparent material, such as glass, overcoated with a conductive material such as tin oxide, copper, copper iodide, gold or the like; however, other suitable materials including many semiconductive materials such as raw cellophane, which are ordinarily not thought of as being conductors but which are still capable of accepting injected charge carriers of the proper polarity under the influence of an applied electric field may be used within the course of the present invention. The use of more conductive materials allows for cleaner charge separation and prevents possible charge buildup on the respective electrode, the latter tending to diminish the interior electrode field. The blocking layer of the imaging electrode, on the other hand, is selected so as to prevent or greatly retard the injection of. charge carriers into the photosenstive pigment particles when the particles reach the surface of this electrode. Although a blocking electrode material need not necessarily be used in the system, the use of such a layer is preferred because of the markedly improved results which it is capable of producing. It is preferred that the blocking layer, when used, be either an insulator or a semiconductor which will not allow for the passage of sufficient charge carriers, under the influence of the applied field, to discharge the particles finely bound to its surface thereby preventing particle oscillation in the system. The result is enhanced image density and resolution. Even if the blocking layer does allow for the passage of some charge carriers to the photosensitive particles it still will be considered to fall within the class of preferred materials if it does not allow for the passage of sutficient charge so as to recharge the particles to the opposite polarity. Exemplary of the preferred blocking materials used are baryta paper, Tedlar, Mylar and polyurethane. Any other suitable materials having a resistivity of from about 10' ohms-cm. or greater may be employed. Typical materials in this resistivity range include cellulose acetate coated papers, cellophane, polystyrene and polytetrafluoroethylene. The core of the blocking electrode generally will consist of a material which is fairly high in electrical conductivity. Typical conductive materials including conductive rubber, and metal foils of steel, aluminum, copper and brass have been found suitable. Preferably, the core of the electrode will have a high electrical conductivity in order to establish the required field differential in the system; however,

if a material having a low conductivity is used a separate electrical connection may be made to the back of the blocking layer of the blocking electrode. For example, the blocking layer or sleeve may be a semiconductive polyurethane material having a conductivity of from about l to 10- ohms-cm. If a hard rubber non-conductive core is used then a metal foil may be employed as a backing for the blocking sleeve. Other materials that may be used in conjunction with the injecting and blocking electrodes and other photosensitive particles which may be used as the photomigratory pigments and the various conditions under which the process operates may be found in the above cited issued patents US. Pat. Nos. 3,384,565 and 3,384,566 as well as US. Pat. Nos. 3,384,- 488 and 3,389,993.

It is to be understood that any suitable photosensitive pigment particle such as identified in the above cited patents may be employed within the course of the present invention with the selection depending largely upon the photosensitivity and the spectral sensitivity desired. Typical photoresponsive organic materials include substituted and unsubstituted organic pigments such as phthalocyanines, for example, copper phthalocyanine; beta form of metal-free phthalocyanine; tetrachlorophthalocyanine; and x-forrn of metal-free phthalocyanine; quinacridones as for example 2,9-dimethyl quinacridone; 4,1l-dimethyl quinacridone; 3,10-dichloro-6,13-dihydroquinacridone; 2,9-dimethoxy-6,1 3-dihydro quinacridone and 2,4,9,1l-tetrachloro-quinacridone; anthraquinones such as 1,5-bis-(betaphenylethylamino) anthraquinone; 1,5-bis-(3'-methoxypr0pylamino) anthraquinone; 1,2,5,6- di (C,C-diphenyl)-thiazole-anthraquinone; 4-(2-hydroxyphenyl-methoxyamino) anthraquinone; triazines such as 2,4-diaminotriazine; 2,4-di-(1'-anthraquinonyl-amino)- 6 '(l" pyrenyl)-triazine; 2,4,6-tri-(1',1",l"-pyrenyl)-triazine; azo compounds such as 2,4,6-tris (N-ethyl-N hydroxy-ethyl-p-aminophenylazo) phloroglucinol; l,3,5,7- tetrahydroxy-2,4,6,8-tetra (N-methyl-N-hydroxy-ethyl-pamino-phenylazo) naphthalene; 1,3,5-tri-hydroxy-2,4,6- tri (3 nitro-N-methyl-N-hydroxy-methyl-4aminophenylazo) benzene; metal salts and lakes of azo dyes such as calcium lake of 6-bromo-1 (l'-sulfo-2-naphthylazo)-2- naphthol; barium salt of 6-cyano-1 (l-sulfo-2-naphthylazo)-2-naphthol; calcium lake of 1-(2-azonaphthaliene- -sulfonic acid)-2-naphthol; calcium lake of 1-(4'-ethyl- -chloroazo-benzene-2-'sulfonic acid)-2-hydroxy-3-naphthoic acid; and mixtures thereof. Other organic pigments include polyvinylcarbazole; tri-sodium salt of 2-carboxyl phenyl azo (2-naphthiol-3,6-disulfonic acid; N-isopropylcarbazole; 3-benzylidene aminocarbazole; 3-arninocarbazole; 1-(4'-rnethyl-5'-chloro-2'-sulfonic acid) azobenzene- 2-hydroxy-3-naphthoic acid; N-Z" pyridyl-8,13-dioxodinaphtho-(2,l-b; 2',3'-d)-furan-6-carboxarnide; 2-amino- S-chloro-p-toluene sulfonic acid and the like.

Typical inorganic photosensitive compositions include cadmium sulfide, cadmium selenide, cadmium sulfoselenide, zinc oxide, zinc sulfide, sulfur, selenium, antimony sulfide, lead oxide, lead sulfide, arsenic sulfide, arsenicseleniurn, and mixtures thereof. The imaging suspension may contain one or more different photosensitive particles each having various ranges of spectral response.

A wide range of voltages may be applied between the electrodes in the system. For good image resolution, high image density and low background it is preferred that the potential applied be such as to create an electric field of at least about 300 volts per mil across the imaging suspension. For example, when the imaging suspension is coated to a thickness of about 1 mil the electrode spacing will be such that an applied potential of about 300 volts produces a field across the suspension of about 300 volts per mil. Potentials as high as 8,000 volts have been applied to produce images of high quality. As is apparent the applied potential necessary to obtain the desired field of strength will vary depending upon the interelectrode gap as well as the type and thickness of the blocking material utilized, The imaging suspension is generally coated to a thickness of up to about 1 mil or 25 microns, with a preferred operational thickness being in the range of from about 3-5 microns.

PREFERRED EMBODIMENTS To further define the specifics of the present invention the following examples are intended to illustrate and not limit the particulars of the present system. Parts and percentages are by weight unless otherwise indicated.

In the following examples, the transparent or NESA electrode consists of a 3% inch diameter Pyrex glass cylinder with a conductive tin oxide coating. The imaging or blocking electrode consists of a 3% inch diameter conductive steel core with A inch layer of polyurethane forming the blocking layer and a paper sleeve.

EXAMPLE I An imaging suspension comprising the beta form of copper phthalocyanine, C.I. 74160, is prepared, 5 parts by weight of the photosensitive pigment being dispersed in about parts mineral oil. To the suspension is added 6 p.p.m. iodine. The suspension is coated on the surface of the NESA electrode to a thickness of about 5 microns. As the blocking electrode with the paper sleeve is passed across the surface of the suspension a potential of about +7,000 volts is applied to the roller. The imaging suspension is exposed to a positive transparency with a visible light source. By a single material transfer a high quality negative image is formed on the surface of the paper sleeve.

EXAMPLE -II The process of Example I is repeated with the exception that the polarities on the blocking and the NESA electrodes are reversed. In this instance there results a high quality positive image on the surface of the paper sleeve thereby demonstrating the capability of the present system to image in a positive to positive imaging mode.

EXAMPLE-S III AND IV The comparative processes of Examples 1 and II are repeated with the exception that bromine is substituted for the iodine additive. A similar result is realized in each instance.

EXAMPLE V A commercial, metal free phthalocyanine is purified by acetone extraction to remove organic impurities. One hundred grams of the resulting beta form of phthalocyanine is dissolved in 600 cc. of sulfuric acid. The resulting solution is poured into 3,000 cc. of ice water and the resulting precipitate washed with water to neutrality. The resulting alpha phthalocyanine is then salt milled for 6 days and then desalted by slurrying in distilled water, vacuum filtering and water washing. The resulting material is methanol washed until the filtrate is clear thereby producing the x-form of phthalocyanine. After vacuum drying to remove residual methanol the x-form phthalocyanine is used to prepare the imaging suspension of the present example by mixing 5 parts by weight of the x-form phthalocyanine pigment in 100 parts mineral oil. To the suspension is added 4 p.p.m. iodine, The resulting suspension is coated on the surface of the NESA electrode to a thickness of about 4 microns and the blocking electrode with the paper sleeve is passed across the suspension. A potential of about +7,000 volts is applied to the roller electrode and the imaging suspension is selectively exposed to a positive transparency with a visible light source. A high quality negative image is formed on the paper sleeve.

EXAMPLE VI The process of Example V is repeated with the exception that the polarities on the electrodes are reversed. There results in this instance a positive image on the surface of the paper sleeve thus further demonstrating the capability of the present system to image both in a positive to negative and a positive to positive imaging mode by merely reversing the polarities on the respective electrodes.

EXAMPLE VII The process of Example I is repeated except a suspension of Red Lake C (Z-amino--chloro-p-toluenesulfonic acid), C.I. 15585, is prepared by dispersing 5 parts by weight of the pigment in 100 parts Sohio 3454 (Odorless kersonene Sohio Oil Co. fraction). To

the suspension is added 8 p.p.m. of iodine. The result-v ing suspension is coated on the surface of the NESA electrode to a thickness of about 5 microns and the blocking electrode with the paper sleeve is passed across the suspension. A potential of about +8,000 volts is applied to the roller electrode as the suspension is selectively exposed to a positive transparency with a visible light source. A high quality negative image is formed on the surface of the paper sleeve.

EXAMPLE VIII The process of Example VII is repeated with the exception that the polarities on the electrodes are reversed. There results in this instance a positive image on the paper again demonstrating the capability of the present system to image both in a positive to negative and positive to positive imaging mode by merely reversing the polarities on the respective electrodes.

EXAMPLES lX-X The process of Examples V and VI are repeated with the exception that 5 p.p.m. of bromine are added in place of 4 p.p.m. of iodine to the suspension. Again the positive to negative and positive to positive imaging capabilities are demonstrated by changing the polarities on the electrodes Although the present examples were specific in terms of conditions and materials used any of the above listed typical materials maybe substituted where suitable in the above examples with similar results. In addition to the steps used to carry out the process of the present invention, other steps or modifications may be used if desirable. For example, the imaging suspension may be pre-charged prior to imaging. In addition, other materials may be incorporated in the imaging suspension, and modifications made to the injecting and blocking electrode which will enhance, synergize or otherwise desirably effect the properties of the system for their present use. For

example, the imaging suspension may contain sensitizers for the photosensitive particles suspended in the liquid carrier.

Those skilled in the art will have other modifications occur to them based on the teachings of the present invention. These modifications are intended to be encompassed within the scope of this invention.

What is claimed is:

1. A method of reversal of photoelectrophoretic imaging which comprises the steps of:

(a) providing a layer of an imaging suspension comprising finely-divided electrically photosensitive particles in an insulating carrier liquid on an injecting electrode;

(b) applying an electrical field across said imaging suspension; and

(c) exposing said imaging suspension to a pattern of electromagnetic radiation to which at least a portion of said particles are responsive until said electrically photosensitive particles migrate away from said injecting electrode in non-exposed areas and are deposited on said injecting electrode in exposed areas, wherein said imaging suspension contains from about 2 to about 200 parts per million of a halogen selected from the group consisting of iodine and bromine.

2. The method of claim 1 wherein said halogen comprises iodine.

3. The method of claim 1 wherein said halogen comprises bromine.

4. The method of claim 1 wherein said electrical field is of a first polarity and a positive image is formed on said injecting electrode.

5. The method of claim 1 wherein said electrical field is of a second polarity and a negative image is formed on said, conductive electrode.

6. The method of claim 1 wherein said injecting electrode is transparent and said suspension is exposed to radiation projected through said electrode.

References Cited UNITED STATES PATENTS 3,041,166 6/1962 Bardeen 961 3,384,566 5/1968 Clark 20418l GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner U.S. Cl. X.R. 96-1 R, 1.3 

